So, yeah, I made a heat sink from angle aluminum, bolted the transistors to it with heat-sink paste and mounted it with nylon screws and spacers.
The system has 2 control boxes, upper and lower. The two control boxes have electrically identical circuitry with DPST momentary toggle switches in the bottom box and DPST momentary push-buttons in the top box.
One pole of the switches powers up a coil for a particular hydraulic valve (6 in all: ram 1 extend, ram 1 retract, ram 2 extend, ram 2 retract, motor rotate clockwise, motor rotates counter-clockwise), to select the function, and the other pole powers up the voltage follower that runs the proportional valve for speed control. There is an on-off-on toggle to power up only one box at a time.
The emitters of the tranistors are connected to their respective coils, which are grounded on the other end.
The collectors are all connected to +12V, unswitched.
Each transistor base goes to a node that splits off to the two control boxes. In the inactive control box, the 6 function conductors just hook to an open switch so there should be no interference.
Because of the passive resistors on each side of the potentiometers, I shouldn’t have connected the bases that way but that’s not what’s causing the problem and I know 2 ways to fix it: either move the resistors from after the pots to after the node, or else put a switch instead of the node, to select which speed control goes to the driver base.
Here’s the problem: with the lower control box connected, the system works beautifully. But with the upper control box connected, the lower control box no longer works right, even though the wires only connect to open switches.
Example: if Rotate Clockwise is operated, then some other function, it will rotate clockwise no matter what other function is selected, over and over again. Unplugging the upper control box wiring brings everything back to normal, the lower control box runs the hydraulics perfectly.
It’s like the coils still have current just because of a dead wire hanging off the base node.
I ohmed out the (brand-new) cable and there are no shorts it. Disconnecting one conductor at a time led to this: with the entire upper control box out of the picture, and the other ends just dangling in the air, it still happens. Various combinations yield different crosstalk patterns but there’s no doubt the long wires themselves are making the trouble.
(If I connect just the upper box and disconnect the lower, electrically same circuit, it doesn’t work right, seems to select multiple functions at once.)
For a final verification, I 1) disconnected all upper control box wiring and ran all the functions with speed control, then 2) connected only the upper box speed control wire to its base node (other end floating in the air) and observed the proportional valve remains open even after the function switch is off and the speed knob back to zero. Repeat 5 times, same result.
It’s like the coil will not discharge if there’s a long open wire tapped on to the base of its driver. Also, it seems the long wires render the upper control box unfunctional.
Next warmish day I will connect the upper control box to the shorter wires of the lower control box just to verify it’s actually the same but the ohm meter says it’s right.
The cable is #18 stranded 10-conductor for the signals and #14 stranded for power and ground strapped alongside, approximately 50 feet long. It only carries a few mA for the voltage follower plus the base currents for the selected function and speed control.
The modified voltage follower current goes from +12V, through a 2K resitstor, a 2K pot, then a 1K resistor. The center tap on the pot goes right to the base of the coil driver.
So, why do the long wires cause this problem? It’s just DC so, how can a coil stay charged up after the switches all open up, but only with the long conductors in the picture? What components can I buy to drain off or soak up whatever charges or reflections or whatever’s locking up the coils?
Thanks, Happy New Year.